The use of ion exchangers, both organic and inorganic, including crystalline molecular sieve zeolites, in order to remove certain metals from aqueous solutions is notoriously old in the art and the patent and technical literature contains many examples of such techniques. Although molecular sieves generally are effective for the removal of certain cations, nevertheless, when competing cations are present in the aqueous solution, a molecular sieve will function normally to the point at which the metal which is desirous of being removed effectively occupies some portion of the ionic sites in said zeolite. Thereafter, the zeolite must either be discarded or regenerated.
A very practical use for the above type of operation is in the home water softening industry wherein an ion exchanger of the organic or inorganic type is contacted with water until the calcium and magnesium ions which are inherently present in most mineral water replaces the ion originally associated with the ion exchanger, usually sodium. At this point, the ion exchanger has to be regenerated and this is usually accomplished by back-washing, or back-flushing, or otherwise contacting the ion exchanger with a solution of a different cation than that which was removed from the water, i.e., usually sodium in the form of sodium chloride. The sodium exchanges for the calcium/magnesium in the spent ion exchanger and the cycle is ready to start anew.
In evaluating the properties of a suitable ion exchanger, it is quite obvious that the environment in which it works to remove the unwanted metal or metals is of extreme importance and its susceptibility to competing ions is of paramount importance in obtaining a practical exchanger as opposed to one that is merely a scientific curiosity.
Thus, for example, in industrial processes wherein heavy metals are present in contaminated aqueous solutions, such heavy metals are not ordinarily present by themselves because the water contains other minerals, particularly calcium and magnesium. Thus for an ion exchanger to be practical in the contact of industrial waste streams containing heavy metals, it is necessary that the ion exchanger be sufficiently selective towards heavy metals versus magnesium or calcium which compete for the ion exchange sites in the zeolite.
Another significant area where ion exchangers can be utilized is in the field of drinking water. The contamination of drinking water by toxic heavy metals, especially lead, has become a topic of great interest in both the scientific and popular press.
The Environmental Protection Agency (EPA) has stated that there is no threshold level of lead below which water is considered safe for human consumption. The EPA estimates that 138 million residents in the United States are potentially at risk from some degree of lead poisoning.
Sources of lead contamination include industrial waste as well as lead-bearing solders and other components found in the plumbing of most homes and water coolers.
There are various techniques utilized for the removal of lead, such as precipitation techniques. Such processes while effective in bulk removal are ineffective in reducing lead levels below about 50 ppb, an unacceptable level for human consumption. Other potential options for lead removal include purification of contaminated streams by exposure to synthetic ion exchange resins or various absorbents such as activated carbon. These systems typically suffer from low dynamic metal capacities, low lead selectivities and unacceptably slow rates of metal removal.
The Environmental Protection Agency has made the radical proposal of reducing the corrosivity of potable water by making all drinking water alkaline with the intent of reducing the leaching of lead from plumbing components. In addition to the high cost and massive social intrusion, the quality and taste of drinking water in this country is expected to decline from what the public has been accustomed to. In addition, it has not yet been proven that this action alone is sufficient to solve the national lead contamination problem.
Cation exchange represents one potential avenue for the removal of many cationic metal species from aqueous systems. As indicated earlier, crystalline inorganic molecular sieves form the basis of hundreds of commercial ion exchange processes. Each individual molecular sieve demonstrates characteristic preferences of selectivities towards certain counterbalancing ions when exposed to mixed solutions and, thus, the separation or isolation of certain cations may be accomplished by the exposure to mixed cationic solutions to specifically tailored molecular sieves
In pending application Ser. No. 07/373,855, previously referred to, there is disclosed and claimed the use of a material designated as ETAS-10 for the removal of heavy metals including lead from aqueous systems. However, what is not specifically taught in said copending application is the use of such a material for the removal of lead from aqueous systems in the presence of competing ions such as calcium and magnesium.
It has now been discovered that certain large-pored molecular sieves of the ETS-10 type, which contain octahedrally active charged sites, demonstrate remarkable rates of uptake for heavy metal species such as lead, cadmium, zinc, chromium, arsenic and mercury which are orders of magnitude greater than prior art absorbents or ion exchangers under the conditions tested which include the presence of competing ions such as calcium and magnesium. The combination of extraordinary lead selectivities, capacity and uptake rates, allows such materials to strip lead from aqueous streams with minimal contact time allowing direct end use in filters for water purification.